Solid-phase sampling device and methods for point-source sampling of polar organic analytes

ABSTRACT

Sampling devices for sampling an aqueous source (e.g., field testing of ground water) for multiple different analytes are described. Devices include a solid phase extraction component for retention of a wide variety of targeted analytes. Devices include analyte derivatization capability for improved extraction of targeted analytes. Thus, a single device can be utilized to examine a sample source for a wide variety of analytes. Devices also include an isotope dilution capability that can prevent error introduction to the sample analysis and can correct for sample loss and degradation from the point of sampling until analysis as well as correction for incomplete or poor derivatization reactions. The devices can be field-deployable and rechargeable.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. patent applicationSer. No. 16/170,185, having a filing date Oct. 25, 2018, entitled“Solid-Phase Sampling Device and Methods for Point-Source Sampling ofPolar Organic Analytes,” which is incorporated herein by reference inits entirety.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under Grant No.DE-AC09-085R22470, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

Accurate detection of the presence or quantity of organic contaminantsin aqueous samples, e.g., ground water samples, presents manydifficulties as a single sample source can carry multiple differentcontaminants with a wide variety of chemistries. Moreover, many organiccontaminants are volatile or quick to degrade, and as such, theirpresence in a sample may be underestimated or missed altogether due toloss or degradation in the time period from the initial sampling to thetesting protocol. This may be particularly problematic for samplesobtained from natural sources, as transportation and storage timebetween sampling and testing can be extensive. Polar organiccontaminants present additional difficulties, as their hydrophilicitymake analysis from an aqueous sample quite difficult.

Attempts have been made to improve aqueous sample testing. For instance,solid phase extraction (SPE) is often used with aqueous samples forpre-concentration procedures to capture and determine thepresence/quantity of organic contaminants. Unfortunately, SPE presentsdifficulties when targeting polar organic analytes and is particularlyproblematic when a sample includes multiple different (e.g., both polarand nonpolar) organic analytes. Moreover, SPE requires the use ofcalibration curves in quantification techniques and includes thepotential of error introduction at many processing points.

A technique that has been developed in an attempt to improvequantitative accuracy of sample analysis is isotope dilution analysis.Isotope dilution analysis involves addition of isotopically labeledanalogues (also termed a spike) into a sample. Using known dataincluding isotopic abundances, concentration and mass of theisotopically labeled spike, mass of the targeted analyte, and samplequantity, the concentration of an analyte can be calculatedmathematically without the use of calibration curves. Isotope dilutionanalysis can correct for many analysis errors introduced due to, e.g.,sample preparation, poor reproducibility, sample loss, low analyterecovery, instrumental drift, matrix effects, and physical or chemicalinterferences. Unfortunately, the spike is combined with the sampleimmediately prior to analysis, and as such, this method may not accountfor analyte loss/degradation that occurs following collection of asample and prior to analysis and, as such, is not suitable for manyapplications including field use.

What are needed in the art are methods and materials that can accuratelyassess the concentration of potential contaminants, and in particularpolar organic contaminants, in an aqueous sample.

SUMMARY

According to one embodiment, disclosed is sampling device that canbeneficially be compact and portable for use in sampling an aqueoussource for one or multiple targeted analytes. A sampling device caninclude a liquid inlet, a first layer downstream of the inlet, a secondlayer downstream of the first layer, and a liquid outlet downstream ofthe second layer. The first layer can include a SPE medium and can alsoinclude a derivatizing agent for a targeted analyte (e.g., a polaranalyte). Reaction between the targeted analyte and the derivatizingagent can form a derivatized analyte. The second layer can also includea SPE medium, which can be the same or can differ from the SPE medium ofthe first layer. The second layer also includes an isotopically labeledanalogue of the derivatized analyte.

Also disclosed are methods for using a sampling device. For instance, amethod can include introducing an aqueous sample into a sampling device.As the sample contacts the first layer, targeted polar analytes in thesample can contact the derivatizing agent and react with the agent toform a derivatized analyte. The derivatized analyte, being less polarthan the targeted polar analyte, can interact with the SPE media of thedevice and be retained on the media at a detectable level. Theisotopically labeled analogue of the derivatized analyte can alsointeract with the SPE media, and the two (the derivatized analyte andthe isotopically labeled analogue) can come to equilibrium with thederivatizing agent and with the SPE media. After the non-retainedportion of the aqueous sample exits the device through the outlet, thedevice can be transported and/or stored for a period of time prior toanalysis. The method can also include eluting retained materials fromthe SPE media following the storage/transport period and analyzing theeluent to determine the concentration of the targeted polar analyte inthe aqueous sample.

Methods for forming the devices are also disclosed. For instance, amethod can include loading a device housing with the first and secondlayers such that the first layer is downstream of the liquid inlet andthe second layer is downstream of the first layer. The first and secondlayers can include first and second SPE media, respectively. A methodcan also include loading the first layer with the derivatizing agent andloading the second layer with the isotopically labeled analogue of thederivatized analyte. In general, the layers can be loaded with thederivatizing agent and the isotopically labeled analogue prior toloading the layers into the device housing. In one embodiment, thedevices can be reusable, and a method can include removing the SPE mediafollowing elution of a sample and reloading the housing with fresh mediafor re-use.

BRIEF DESCRIPTION OF THE FIGURES

A full and enabling disclosure of the present subject matter, includingthe best mode thereof to one of ordinary skill in the art, is set forthmore particularly in the remainder of the specification, includingreference to the accompanying figures in which:

FIG. 1 schematically illustrates one embodiment of a sampling device.

FIG. 2 illustrates one embodiment of an isotope dilution analysisprofile.

FIG. 3 schematically illustrates another embodiment of a samplingdevice.

FIG. 4 schematically illustrates another embodiment of a samplingdevice.

FIG. 5 schematically illustrates one method for utilizing a samplingdevice.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thedisclosed subject matter, one or more examples of which are set forthbelow. Each embodiment is provided by way of explanation of the subjectmatter, not limitation thereof. In fact, it will be apparent to thoseskilled in the art that various modifications and variations may be madein the present disclosure without departing from the scope or spirit ofthe subject matter. For instance, features illustrated or described aspart of one embodiment may be used in another embodiment to yield astill further embodiment.

In general, disclosed are sampling devices that can be beneficiallyutilized in one embodiment for sampling an aqueous source (e.g., fieldtesting of ground water) for multiple different analytes. The device canbe small and portable, as well as disposable or re-chargeable, and canstore a sample for a period of time (e.g., during transport from asource and prior to analysis) and still provide accurate analysis of theanalyte concentration in the sample.

More specifically, disclosed devices provide for targeted analytederivatization and solid-phase extraction in conjunction with isotopedilution in a single, portable device. The SPE component of the devicesprovides for retention of a wide variety of targeted analytes. Theanalyte derivatization capability allows for extraction of polarcompounds simultaneously with nonpolar compounds, and by such, a singledevice can be utilized to examine a sample source for a wide variety ofanalytes. The isotope dilution aspect of the devices can prevent errorintroduction to the sample analysis and can correct for sample loss anddegradation from the point of sampling until analysis, allowing forsamples to be stored for long periods of time. Moreover, the isotopedilution aspect can correct for incomplete or poor derivatizationreactions. Thus, the devices can be field-deployable and can provideaccurate analysis of any water source, no matter how remote, for a widevariety of analytes, and in particular, for potential contaminants.

Polar organic analytes are notoriously difficult to accurately assess inaqueous samples, and disclosed devices can solve many problemsassociated with these materials. FIG. 1 schematically illustrates oneembodiment of a device for use in sampling a polar organic analyte 10 asmay be found in an aqueous sample. A device can include a housing 2, aswell as an inlet 4 and an outlet 6. While there is no particularlimitation on the size of a device, in one embodiment, the housing 2 canbe relatively small so as to be compact and portable for sampling in thefield. For instance, the housing 2 can be cubic, cylindrical, or anyother convenient shape, and in one embodiment, can have a width anddepth of from about 1 inch to about 6 inches and a height of from about3 inches to about 10 inches. The material of formation of the housing isnot particularly limited, though in one embodiment, the housing can beformed of a plastic material that can provide resiliency and lightweight to the device.

The inlet 4 and outlet 6 can include closures and one or both of theinlet 4 and the outlet 6 can be permanently or removably attachabletubing, which can allow for control of the liquid flow into and/or outof the housing. For instance, the outlet 6 can connect to plastic tubing8 that can in turn be connectable to a vacuum pump 10 that can be usedin those embodiments in which organic materials retained within thehousing 2 are dried following collection from an aqueous sample andprior to transport, storage, and/or analysis.

A first layer 20 and a second layer 30 are located within the housing 2.As shown, the layers 20, 30 are located within the housing 2 such that asample introduced to the device via inlet 4 will contact the first layer20 prior to contacting the second layer 30.

The first layer 20 includes a first SPE media 22 and the second layer 30includes a second SPE media 32. Depending upon the nature of the SPEmedia, the layers 20, 30 can also include suitable retention devices forthe SPE media. For instance, in those embodiments in which the SPE mediaare in the form of porous particulates, the layers can include retentiondevices, such as wire or plastic mesh or the like, that can retain theSPE media in the desired location within the housing. Adhesives or thelike that do not prevent desired association between a sample and theSPE media can also be used to retain the SPE media in a defined layerwithin the housing, for instance in the form of a removable cartridgethat can be located and retained at a predetermined location within thehousing.

The SPE media 22, 32 can be the same or can differ from one another andcan vary depending upon the nature of the analytes targeted forretention by use of the device. For instance, in those embodiments inwhich an aqueous sample is to be analyzed for polar and/or nonpolarorganic analytes, the SPE media 22, 32 can include any solid phaseextraction sorbent that can be utilized for a range of polar andnon-polar compounds. Examples of suitable solid phase extractionsorbents include, without limitation, carbon-based media such as porousparticular carbon molecular sieves (e.g., Carbosieve® absorbents),graphitized polymer carbon (e.g., spherical graphitized polymer carbon),graphitized carbon black (GCB), pyrocarbon reinforced GCB; silica-basedmedia; etc.

In addition to the SPE media 22, the first layer 20 also includes aderivatizing agent 23. The derivatizing agent 23 can be selected forreaction with a targeted analyte 10 such that upon interaction, reactionbetween the derivatizing agent 23 and the targeted analyte 10 can form aderivatized analyte 24, as indicated in FIG. 1 .

Analytes as may be collected and analyzed by use of disclosed devicesare not particularly limited and can generally include any analyte ofinterest as may be found in an aqueous sample. In general, the targetedanalytes will encompass relatively low molecular weight organicanalytes, e.g., having a number average molecular weight of about 500g/mol or less, about 250 g/mol or less, or about 100 g/mol or less insome embodiments. In one embodiment, targeted analytes can encompasspotential contaminants of ground water. Exemplary targeted analytes caninclude, without limitation, perchloroethylene, tetrachloroethylene,benzene, toluene, xylene, ethylbenzene, polychlorinated biphenylisomeric congeners, and halogenated pesticides and herbicides includingalachlor, atrazine, bromacil, cyanazine, endrin, heptachlor,metolachlor, and chlorpyrifos.

Basic derivatizing reactions can include, for example, silylation,acylation, and alkylation, with the preferred derivatizing reactiondepending upon the polarity of the targeted analyte and that of thereaction product as well as the chemical nature of the SPE media. Ingeneral, it can be desired that the reaction product be less polar thanthe analyte, and as such will exhibit a better solid-liquid distributioncoefficient with the SPE medium, i.e., the derivatized analyte will beretained by the SPE media with higher preference as compared to thenon-derivatized targeted analyte.

Derivatizing agents can also be selected through determination of theease/speed of derivatization of the targeted analyte by the derivatizingagent. For instance, when considering a silylation derivatization,alcohols are much more efficiently silylized as compared to amide, andas such, if the targeted analyte is an alcohol, it may be preferred toselect a silylating derivatizing agent. However, when selecting aderivatizing agent for an amide-containing targeted analyte, it may bepreferred to select a different derivatizing agent that is moreefficient for the amide-containing targeted analyte, e.g., an acylatingderivatizing agent.

Silylating derivatizing agents can be selected for derivatization oftargeted analytes including, without limitation, alcohols, amines,amides, aldehydes, thiols, phenols, enols, and carboxylic acids.Examples of silylating derivatizing agents can include, withoutlimitation, alkylsilanes or arylsilane such as derivatives oftrimethylsilyl-, t-butyl dimethyl silyl or other alkylsilyl- orarylsilyl agents. Specific examples of silylating derivatizing agentscan include, without limitation,N-methyl-N-(trimethylsilyl)trifluoroacetamide,N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide,1,3-bis(chloromethyl)-1,1,3,3-tetramethyldisilazane,1-(trimethylsilyl)imidazole, 3-trimethylsilyl-2-oxazolidinone,allyl(chloro)dimethylsilane, bromotrimethylsilane, chlorotriethylsilane,chlorotriisopropylsilane, chlorotrimethylsilane, hexaethyldisiloxane,hexamethyldisilazane, N,N′-bis(trimethylsilyl)urea,N,N-dimethyltrimethylsilylamine, N,O-bis(trimethylsilyl)acetamide,N-methyl-N-trimethylsilylacetamide,N-methyl-N-trimethylsilylheptafluorobutyramide, trimethylsilylmethanesulfonate, trimethylsilyl N,N-dimethylcarbamate, trimethylsilyltrifluoromethanesulfonate, triphenylsilane, methyl3-trimethylsiloxy-2-butenoate, phenylchlorosilane, or triethylsilane ormixtures thereof.

Acylation can be selected for derivatization of targeted analytesincluding, but not limited to, amines, amides, alcohols, thiols,phenols, enols, glycols, unsaturated compounds, and aromatic rings.Examples of acylating derivatizing agents can include, withoutlimitation, acid anhydrides, acid halides, reactive acyl derivativessuch as acrylated imidazoles, acrylated amides, alkali metal salts ofcarboxylic acids, and acrylated phenols. Specific examples of acylatingderivatizing agents can include, without limitation, acyl chlorides orother acyl halides, acetic anhydride, propionic acid anhydride, mixedanhydrides of acetic and propionic acids, acetyl chloride, butyric acidchloride, benzoyl chloride, propionic acid chloride, stearyl chloride,alkali metal salts of carboxylic acids having between two and eightcarbon acids, nitrophenyl carbonate, trichlorophenyl carbonate,pentachlorophenyl carbonate, and carbonyl imidazole, as well as variousactive esters, e.g. nitrophenyl ester, pentafluoroethyl ester,trichlorophenyl ester.

Alkylating derivatizing agents can be selected for derivatization oftargeted analytes including, without limitation, carboxylic acids,amines, amides, alcohols, thiols, phenols, and enols. Examples ofalkylating derivatizing agents can include, without limitation, alkylhalides, dialkyl sulfates, nitro-substituted chloro- or fluoro-benzenes,and alkylammonium salts. Specific examples of alkylating derivatizingagents include, without limitation, benzyl chloride, methyl chloride,alpha-chloroacetic acid, dimethyl sulfate, alpha-chloromethyl phosphoricacid, tetraalkylammonium hydroxides, dimethylformamide dialkyl acetals,and diazoalkanes.

Referring again to FIG. 1 , upon the derivatization reaction in thefirst layer 20 of the device, the liquid sample will flow to contact thesecond layer 30 of the device. The second layer 30 contains anisotopically labeled analogue 25 of the derivatized analyte 24. In oneembodiment, the isotopically labeled analogue can be a carbon-13 labeledanalogue, but the devices and methods are not limited to carbon-13labeled analogues, and any stable analogue of the derivatized analytecan be utilized, including but not limited to deuterium labeledanalogues. The derivatized analytes, the isotopically labeled analogues,and the derivatizing agents of the device can come into equilibrium withone another within the device. The isotope of the isotopically labeledanalogue will generally not be in a position on the analogue where itcan interact or react with the derivatizing agent, as it (the isotope)could then be lost via a reverse reaction during equilibration.

Upon interaction of the liquid sample with both of the first layer 20and the second layer 30, the two layers can act upon one anothersynergistically to induce in-phase equilibration of the derivatizedanalyte 24 and the isotopically labeled analogue 25. As is known,derivatization can be imprecise and incomplete and materials retained onSPE media can degrade over time. As such, following the initialderivatization interaction and equilibrium between the variouscomponents and the SPE media, a portion of the targeted analyte in thesample, as well as a portion of the isotopically labeled analogue, canbe lost as the sample flows through the device during sample collectionas well as during transport/storage of the device prior to elution ofthe retained materials from the SPE media and analysis of the eluent.However, as the loading level of the isotopically labeled analogue isknown, and as the isotopically labeled analogue and the derivatizedanalyte will be subject to the same conditions from the time of samplinguntil the time of analysis, determination of the loss (if any) of theisotopically labeled analogue in this period can be used tomathematically correct the analysis results regarding the concentrationof targeted analyte in the original sample due to e.g., incompletederivatization, sample loss, degradation, volatility, interferents, etc.

For instance, and with reference to FIG. 2 , m/z standards for a naturalcompound (¹²C-R) and an isotopically labeled analogue (¹³C-R) are shown,which demonstrate the relative amounts of the carbon-12 compound and thecarbon-13 compound for each material. The lower graph of FIG. 2illustrates an exemplary m/z result as may be obtained upon masschromatography analysis of a sample following elution from a device thatincludes both the natural compound and the isotopically labeled spike.As the initial quantity of the isotopically labeled analogue loaded intothe device is known, degradation/loss of the isotopically labeledanalogue in the eluent can be determined from the analysis results forthe isotopically labeled analogue. This information can then be utilizedto apply a correction factor to the analysis results for the derivatizedanalyte, so as to obtain an accurate concentration of the targetedanalyte in the initial sample.

Devices can be utilized to determine the concentration of severaldifferent analytes in a sample, even when those analytes exhibitdifferent chemistries. For instance, FIG. 3 illustrates one embodimentof a device that can be utilized in determining the concentration of afirst targeted polar organic analyte 10 as well as a nonpolar organicanalyte 40. As the nonpolar organic analyte 40 can already exhibit goodretention characteristics on the SPE media 22, 32, the device need notinclude a derivatization agent for the nonpolar organic analyte.However, to improve quantitative detection of the nonpolar organicanalyte in a sample, the device can include an isotopically labeledanalogue 45 of the nonpolar organic analyte 40. This isotopicallylabeled analogue 45 can be loaded into a device in a known quantity andas such, can be utilized to determine a correction factor for any lossor degradation of the nonpolar organic analyte 40 from the time ofcollection until the time of elution and analysis.

In another embodiment, a device can be designed to capture and analyzethe concentration of one or more additional polar and/or nonpolarorganic analytes in a sample. For instance, as illustrated in FIG. 4 , adevice can be designed to capture both a first targeted analyte 10 and asecond targeted analyte 50, both of which being polar organic analytes.The device can be designed to also capture a nonpolar organic analyte 40(or multiple nonpolar organic analytes, as desired). As such, inaddition to the components of the device discussed previously(derivatizing agent 23 for the first targeted analyte 10, isotopicallylabeled analogue 25 of the derivatized analyte 24, isotopically labeledanalogue 45 of the nonpolar organic analyte 40), the device can includea second derivatizing agent 53 loaded in the first layer 20. The secondderivatizing agent 53 can be selected for derivatization of the secondtargeted analyte 50. As shown, reaction of the second targeted analyte50 with the second derivatizing agent 53 can form a second derivatizedanalyte 54. The second derivatizing agent 53 can be the same or candiffer from the derivatizing agent 23 used for the first targetedanalyte 10, generally depending upon the nature of the targetedanalytes.

The device of FIG. 4 can also include an isotopically labeled analogue55 that is an analogue of the second derivatized analyte 54. Asdescribed previously, the presence of a known quantity of theisotopically labeled analogue 55 loaded into the second layer 30 of thedevice can be used to mathematically correct the analysis resultsregarding the concentration of the second targeted analyte 50 in thesample. Thus, upon analysis of the retained materials on the device, anaccurate concentration of all of the targeted analytes can be obtained.

FIG. 5 schematically illustrates one embodiment of a method for usingthe sampling devices. As shown, a device 60 can be utilized for fieldsampling, e.g., at a surface pond 62, as shown. The device 60 can bepre-loaded with the first and second layers and the various derivatizingagents and isotopically labeled analogues, as discussed above. Possiblestorage time for a device prior to loading with the layers and activeagents and prior to use can generally depend on the exact analytes andderivatizing agents used, but in general, a device can provide increasedaccuracy if utilized within a few weeks (e.g., about a month) offormation.

An aqueous sample is collected and passed through the device at step 1,during which the various components can come to equilibrium with oneanother and the sample components that are not retained in the deviceare returned to the sample source at step 2. Thus, the bulk of theliquid of the sample is left at the source while the targeted analytesare retained in the device. Optionally, the retained components can bedried (e.g., by use of a vacuum pump) and the dried, loaded device canbe transported and optionally stored (step 3) prior to elution andanalysis via, e.g., mass spectrometry (step 4). Elution can be carriedout with typical solvents, with the preferred solvent for any protocolgenerally depending upon the particular analytes and derivatizing agentsinvolve. By way of example, solvents can include, without limitation,hexane, methylene chloride, chloroform, diethyl ether, ethyl acetate,acetone, acetonitrile, isopropanol, methanol, and acetic acid.Following, the eluent can be analyzed via mass spectrometry to identifythe isotopes of the various components.

In one embodiment, the device can be rechargeable, in which case theused media (e.g., self-supporting media cartridges) can be removed fromthe housing and the housing can be re-loaded with fresh media that hasbeen charged with derivatizing agents and isotopically labeled analoguesfor re-use.

Beneficially, the device can be small (e.g., easily carried by hand),lightweight, cost-effective, disposable or re-usable, and provide highlyaccurate data with regard to analyte concentration in an aqueous samplesource. In one embodiment, the device can be effectively used byresearchers or industry involved in sampling in locations where thetransport of large amounts of sample source would be cumbersome andinefficient as well as in more accessible locations for relativelysimple, inexpensive, and accurate water sampling (e.g., household use).For instance, following collection of a sample, a device can be sealedand transported via shipping to a testing location, with the testingresults then returned to the sender. By way of example, a device can beused for EPA certification of a water source under several EPA approvedtesting methods (e.g., Methods 6800, 1624, 1625).

While certain embodiments of the disclosed subject matter have beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the subjectmatter.

What is claimed is:
 1. A sampling device comprising: a liquid inlet; afirst layer downstream of the liquid inlet, the first layer comprising afirst solid phase extraction medium, the first layer further comprisinga first derivatizing agent for a first targeted analyte, reactionbetween the first derivatizing agent and the first targeted analyteforming a first derivatized analyte; a second layer downstream of thefirst layer, the second layer comprising a second solid phase extractionmedium, the second layer further comprising an isotopically labeledanalogue of the first derivatized analyte; and a liquid outletdownstream of the second layer.
 2. The sampling device of claim 1,wherein the first solid phase extraction medium and/or the second solidphase extraction medium comprises a carbon-based medium, the first solidphase extraction medium and the second solid phase extraction mediumbeing the same or differing from one another.
 3. The sampling device ofclaim 1, wherein the first derivatizing agent comprises a silylationderivatizing agent, an acylation derivatizing agent, or an alkylationderivatizing agent.
 4. The sampling device of claim 1, wherein theisotopically labeled analogue of the first derivatized analyte is acarbon-13 labeled analogue.
 5. The sampling device of claim 1, whereinthe first layer and the second layer are components of first and secondself-supporting removable cartridges, respectively.
 6. The samplingdevice of claim 1, further comprising a housing that contains the firstlayer and the second layer, the housing having a width of from about 1inch to about 6 inches, a depth of from about 1 inch to about 6 inches,and a height defined between the liquid inlet and the liquid outlet offrom about 3 inches to about 10 inches.
 7. The sampling device of claim1, the first layer further comprising a second derivatizing agent for asecond targeted analyte, reaction between the second derivatizing agentand the second targeted analyte forming a second derivatized analyte,the second layer further comprising an isotopically labeled analogue forthe second derivatized analyte.
 8. The sampling device of claim 1,further comprising an isotopically labeled analogue for a third targetedanalyte in the second layer.
 9. A method for forming the sampling deviceof claim 1 comprising: locating the first layer in a housing downstreamof the liquid inlet, the liquid inlet being defined in a portion of thehousing; and locating the second layer in the housing downstream of thefirst layer; wherein the housing defines the liquid outlet downstream ofthe second layer.
 10. The method of claim 9, wherein the first layer andthe second layer are removably held within the housing.
 11. The methodof claim 9, further comprising loading the first derivatizing agent ontothe first layer prior to locating the first layer in the housing. 12.The method of claim 9, further comprising loading the isotopicallylabeled analogue of the first derivatized analyte onto the second layerprior to locating the second layer in the housing.
 13. The method ofclaim 9, the first layer further comprising a second derivatizing agent,reaction between the second derivatizing agent and a second targetedanalyte forming a second derivatized analyte, the second layer furthercomprising an isotopically labeled analogue of the second derivatizedanalyte.
 14. The method of claim 13, the second layer further comprisingan isotopically labeled analogue of a third targeted analyte.